THERE is a need for a more confined and precise use of the word ribosome. It would clarify molecular biology if the word were reserved for particles in the cell which measure 100 A–300 A in diameter, are composed of RNA and protein, and have been shown to be active as sites for the synthesis of proteins. However, such criteria are not easily available to, say, an electron microscopist. This communication describes an attempt to compare the number of ribosomes in a cell, as calculated from biochemical data, with the number of particles in the same cell type, visible in the electron microscope after osmium tetroxide fixation, and, in terms of size and distribution, categorized as ribosomes in contemporary electron microscopy.